Seconded. Quad-core is only awesome if the program/OS can take advantage of it. As of this point I can only think of 3D programs and FCP that do.
Not even FCP, just Compressor and DVD Studio Pro. FCP still uses RAM, and isn't much of a hog on the cores even when editing HD. Motion and Color are very GFX card intensive.
Compressor and DVD use the cores for encoding, and are so far the only apps that benefit from the multiple cores.
I'd grab the Dual 2.0 or the Quad 2.5 if I had a choice. The last G5 revisions Apple made were single chip dual core 2GHz and 2.3GHz and dual chip dual core 2.5GHz.
These models run a lot quieter, are all water cooled AFAIR, take up to 16GB of RAM (which is DIRT cheap now) use PCIe slots, are are literally the fastest G5 Apple machines made. There isn't much of a difference in speed between the 2.0 and 2.3 which is why i'd grab the cheaper 2.0 or quad 2.5.
my rule of thumb is, "Every year a machine is out on the market the price is decreased by a third." Lots of people will try to pull that "Mac hold their resale value" bu!!$h!t on a G5 PPC, but it's just not the same since Intel came.
If you want a Dual 2.0 ($2000) after 3 years you should pay no more than $600 or so. Even if it's fully loaded with 1TB HDDs and 4GB of Ram since all of those parts are CHEAP NOW. No more than $600 period.
The quad ($3300) if you can find one since they are very very rare and still wonderfully fast machines should be around $1000-1100 as you can see.
With the rumors that Snow Leopard will drop PPC support, you may want to wait until then to buy since the Rev A Mac Pro will be around $1500-$1700 to acquire, and high end users will be dumping their quad G5's to get an OS update that many say are really worth it in multicore computing.